The speed of injecting plasticized synthetic resin into a mold cavity influences the resin flow rate in the cavity. The resin flow rate or flow state determines the surface quality of a molded product. In addition, the flow rate of plasticized synthetic resin in a mold cavity must be suitable to the inside shape of the cavity or an intended molding shape, the resin property, and the molding production efficiency. Therefore, it is necessary to change the resin injection speed in multiple stages in each injection process, so as to obtain a resin flow suitable to the various conditions in the mold cavity. Simultaneously, in order to maintain a constant quality of molded products, it is necessary to control the injection speed at a high accuracy so that the specified amount of plasticized synthetic resin is injected at each selected speed into the cavity in each injection process.
In controlling the resin injection speed varied in multiple stages so as to obtain moldings with a stable quality, it is essential that the injection speed and the amount of resin injected at each selected speed in each injection process have reproducibility. Various methods have been proposed for such control of the injection process. Among them is an art disclosed in the Japanese Patent Publication No. 59060 of Showa 57 (1982).
The injection speed of plasticized synthetic resin, which influences the flow rate of the resin in a mold cavity, is determined by the configurations of the resin flow passages in the mold cavity and injection molder nozzle, the viscosity of the resin, and the pushing pressure applied to the screw. The configurations of the resin flow passages in the mold cavity and injection molder nozzle are fixed. The resin viscosity can be stabilized easily by maintaining a constant resin temperature. The pushing pressure to be applied to the screw can also be controlled easily by means of various controlling devices. Therefore, it is easy to reproduce the injection speed for each stage in each injection process.
According to the conventional injection process control method in which injection speed is varied in multiple stages during each injection process, a constant amount of plasticized synthetic resin is injected at each selected speed into a mold cavity by controlling the injection speed selecting point according to the travel distance of the screw.
In injection molding operation, measuring process is always conducted prior to the plasticized synthetic resin injection into a mold. In this measuring process, a specified amount of plasticized synthetic resin is supplied into the heated cylinder and melted. The screw position at the end of this measuring process is the start point for the screw to travel, that is, the injection start point. Therefore, when the screw position at the end of the measuring process has changed or has been changed, and when the screw is moved over a specified distance to an injection speed selecting point at which the injection speed is changed, it is only possible to control the volume of plasticized synthetic resin injected by the screw moving from the travel start point or injection start point to the injection speed selecting point, but not possible to maintain a constant filling weight, as long as the plasticized synthetic resin in front of the screw is compressible. Not only when the screw position at the end of the measuring process in the initial stage of molding operation has been changed as mentioned above, but also when the pressure applied to the screw immediately before injection and/or the injection speed have been changed, it is not possible to obtain injection speed selecting points which ensure the same filling weight as that before the injecting condition is changed, unless the screw stroke between the injection start point and each injection speed selecting point is changed. Thus, by the conventional injection process control method which determines injection speed selecting points according to the screw traveling distance, it is not possible to detect the injection speed selecting points which ensure a constant filling weight of plasticized synthetic resin by each injection process. Consequently, constantly high quality moldings cannot be produced.
In view of the above problem of the conventional art, the object of the present invention is to provide a method of controlling the injection speed selecting points of an injection molder whose injection speed of plasticized synthetic resin is varied in multiple stages in each injection process, so that the resin is injected by a constant weight from the injection molder cylinder into the cavity of a mold in each stage of each injection process.